Shutter operating circuit having means to close shutter when light is insufficient

ABSTRACT

In a shutter operating circuit comprising a shutter time determining circuit including a photo-electromotive element, and a high input impedance transistor; means to initiate the operation of the shutter time determining circuit; a shutter control circuit controlled by the shutter time determining circuit; and a source of supply connected to said two circuits, there are provided a series circuit connected across the source and including serially connected resistor and capacitor, and a switching transistor with its base electrode connected to the juncture between the resistor and the capacitor, the series circuit acting to trigger the switching transistor to operate the shutter control circuit when the shutter time determining circuit fails to operate after elapse of a predetermined interval after the operation of the initiating means.

United States Patent Yoshimura et a].

[ 1 June 27, 1972 l/l969 Mori et al. ..95/l0CT 12/1969 Fuwa et a1..250/2l2 X Primary Examiner-James W. Lawrence [72] inventors: Hirof'umiYoshimura, Okaya; Hisatoshi Ass[s,ammmine, 1- Grigsby fakaham,shlmosuwa'Mach" both of AttameyChittick, Pfund, Birch, Samuels &Gauthier apan I [73] Assignee: Yashica Company Limited, Tokyo, Japan[57] ABSTRACT [22 Fil d; No 4 1970 in a shutter operating circuitcomprising a shutter time determining circuit including aphoto-electromotive element, and a [2n Appl' 86653 high input impedancetransistor; means to initiate the operation of the shutter timedetermining circuit; a shutter control 30 F i Application priority Datacircuit controlled by the shutter time determining circuit; and a sourceof supply connected to said two circuits, there are NOV. 8, 1969 Japan..44/105631 p i a Series circuit Connected across the Source and cludingserially connected resistor and capacitor, and a if zs switchingtransistor with its base electrode connected to the E g 0 juncturebetween the resistor and the capacitor, the series ciro arc cuit actingto trigger the switching transistor to operate the shutter controlcircuit when the shutter time determining cir- [56] References Citedcuit fails to operate after elapse of a predetermined interval U DSTATES PATENTS after the operation of the initiating means.

3,326,103 6/1967 Topaz ..95/l0 CT 4 Claims, 1 Drawing Figure --L SHUTTERTIME DETERMINING CIRCUIT I 6? 8 RI}? EH3? 4 P O SHUTTER Y CONTROL S2 lCIRCUIT TR-2 TR-l R2 E 2R5 Cl VR I PKTENTEDJUHZT I972 3673.415

A B SHUTTER TIME DETERMINING CIRCUIT Mr RI gm? SHUTTER L CONTROL {S2CIRCUIT INVENTOR S HIROFUM I YOSHIMURA HISA'IOSHI TAKAHATA SHUTTEROPERATING CIRCUIT HAVING MEANS TO CLOSE SHU'I'IER WHEN LIGHT IS INSUIFICIENT BACKGROUND OF THE INVENTION This invention relates to a shutteroperating circuit for cameras, and more particularly to a shutteroperating circuit employing a photo-electromotive element.

1 An electric shutter operating circuit functions to determine a propershutter time in accordance with the brightness of an object to bephotographed, but the prior shutter. operating circuit is defective inthat it operates to continuously maintain the shutter in the openedcondition when the object is very dark. In the prior art shutteroperating circuit utilizing a photoconductive element as means fordetermining the shutter time, for the purpose of eliminating thisdifirculty, a high resistance resistor is connected in parallel with thephotoconductive element so as to establish a definite time constant by acapacitor and the resistance valve of the resistor m manifested therebywhen the photoconductive element manifests an extremely high resistancewhereby to close the shutter after it has been opened for a definiteinterval of time. Such idea has been disclosed in, for example,specifications of the US. Pat. Nos. 3,326,103 and 3,422,738.

When the photoconductive element or the time constant determiningelement is a passive element, the high resistance element connected inparallel therewith operates as desired whereas when the time constantdetermining element comprises a photo-electromotive element it isimpossible to close the shutter with the prior shutter operating circuitutilizing a parallel connected high resistance element.

In. the accompanying drawing blocks A and B show a conventional shutteroperating circuit utilizing a photo-electromotive element. 'Moreparticularly, A represents a shutter time determining-circuit whileblock B a well known shutter control circuit. Since the construction ofthe shutter control circuit contained in block B is well known in theart, the detail thereof has been omitted fromthe drawing a source E isconnected to the shutter operating circuit through a switch 8,. Theshutter time determining circuit A comprises a photoelectromotiveelement P which generates an electromotive force when it is irradiatedwith light andcooperates with a capacitor C to form a serially connectedtime constant circuit. A switch S connected in parallel with capacitor Cis arranged to be opened concurrently with the operation of a shutterrelease. There is'provided a semiconductor element TR-l a field efiecttransistor for example, having a high input impedance characteristic.The source electrode of the field effect transistor TR-l is connected tothe negative pole of the source E through a variable resistor VRwhereasthe drain electrode to the positive pole of the source through avariable resistor R,, a bleeder circuit comprising a resistor R and avariable resistor R is connected between the positive pole of the sourceE and the source electrode, and the juncture between these resistors isconnected to one side of the photoelectromotive element P. The gateelectrode of the field effect transistor TR-l is connected to thejuncture between the element P and capacitor C In the circuitconstructed as above described, in order to eliminate the effect of thedark current upon the switching time it is necessary to decrease thenegative bias impressed upon the photo-electromotive element P, forexample a solar cell, thus decreasing the dark current. The presentlyavailable solar cell having a high signal-to-noise ratio has a verysmall output, moreover there is a limit for the signal-to-noise ratioand the internal resistance of the solar cell is very high, for examplemore than several thousand megohms. It is impossible to preventpermanent opening of the shutter even when a high resistance resistor isconnected in parallel with the photo-electromotive element.

SUMMARY OF THE INVENTION It is the principal object of this invention toobviate aforementioned defects.

More specific object of this invention is to provide a novel shutteroperating circuit which can positively close the shutter of a cameraeven when the brightness of an object is not sufficiently high.

According to this invention, in a shutter operating circuit comprising ashutter time determining circuit including a photo-electromotive elementand a high input impedance transistor; means to initiate the operationof the shutter time determining circuit; a shutter control circuit.controlled by the shutter time determining circuit; and a source ofsupply connected to two circuits described above, there are provided aseries circuit including a resistor and a capacitor which is connectedacross the source and a switching transistor with its base electrodeconnected to the juncture between the resistor and the capacitor, theseries circuit acting to trigger the switching transistor to operate theshutter control circuit when the shutter time determining circuit failsto operate after elapse of a predetermined interval after the operationof the initiating means.

BRIEF DESCRIPTION OF THE DRAWING The invention can be more fullyunderstood from the following detailed description taken in conjunctionwith the accompanying drawing in which a single FIGURE shows a blockdiagram of one example of the shutter operating circuit embodying thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to this invention, aseries circuit including a resistor R and a capacitor C is connectedacross E source through switch S and the juncture between resistor R andcapacitor C is connected to the base electrode a switching transistorTR-2. The emitter electrode of this transistor is connected to thenegative pole of source E through a potentiometer resistor R whereas thecollector electrode of transistor TR-2 is connected to the drainelectrode of the field effect transistor TR-l.

Resistor R and capacitor C comprises a time constant circuit thatdetermines an instant at which switching transistor TR-2 becomesconductive, the time constant being determined by a preset intervalbetween the opening of the shutter caused by a release button and aninstant at which the shutter should be closed. The time constant isdetermined by the product of the resistance of resistor R and thecapacitance of capacitor C In operation, switch S is closed.Concurrently with the operation of the shutter release, short circuitingswitch S is opened. The output current from the photo-electromotiveelement P commences to charge capacitor C Where the light impinging uponthe photo-electromotive element P is extremely weak so that the voltageacross capacitor C does not build up to a valve suflicient to turn ONtransistor TR-l, this transistor will not be rendered conductive by thecapacitor voltage so that the shutter will be maintained in the openedstate. However, according to this invention, capacitor C is charged bysource E through switch S, and resistor R to a voltage sufficient torender conductive transistor TR-2 during the predetermined interval. Asa result, transistor TR-2 controls the shutter control circuit B as ifthe field effect transistor TR-1 were rendered conductive, thus closingthe shutter.

In accordance with another embodiment of this invention, the intervalwhich is determined by the resistor R and the capacitor C, can be madevariable. An example of this construction is provided, as shown in thedrawing by connecting another resistor R through a switch S across theresistor R When the switch S, is closed, the time constant obtained bythe resultant resistance of the resistor parallel circuit and thecapacitor C becomes decreased and therefore said interval becomes short.Such a shutter operating circuit as this embodiment is useful for anautomatic shutter camera for use with photoflash device.

Thus, this invention provides a new and improved shutter control circuitwhich assures desired shutter opening period under adverse lightconditions.

It will be clear that the invention is not limited to the particularembodiment illustrated and that many changes and modifications willoccur to one skilled in the art within the true scope and spirit of theinvention as defined in the ap pended claims.

What is claimed is:

1. In a shutter operating circuit comprising a shutter time determiningcircuit including a photo-electromotive element and a high inputimpedance transistor; means to initiate the operation of said shuttertime determining circuit; a shutter control circuit controlled by saidshutter time determining circuit; and a source of supply connected tosaid two circuits, the improvement which comprises a series circuitconnected across said source and including serially connected a firstresistor and capacitor and a switching transistor with its baseelectrode connected to the juncture between said resistor and capacitor,said series circuit acting to trigger said switching transistor tooperate said shutter control circuit when said shutter time determiningcircuit fails to operate after elapse of a predetermined interval afterthe operation of said initiating means.

2. The shutter operating circuit according to claim 1 wherein saidphoto-electromotive circuit is connected in series with a capacitorwhich is normally short circuit by a switch which is opened when ashutter release is operated so that when an object to be photographedhas sufficient light, said photo-electromotive element charges saidcapacitor within a predetermined interval to trigger said high inputimpedance transistor.

3. The shutter operating circuit according to claim 1 wherein said firstresistor is a variable resistor.

4. The shutter operating circuit according to claim 1 is furtherprovided with the second resistor connectable across said first resistorof said series circuit and a switch adapted to connect said secondresistor in parallel with said first resistor.

1. In a shutter operating circuit comprising a shutter time determiningcircuit including a photo-electromotive element and a high inputimpedance transistor; means to initiate the operation of said shuttertime determining circuit; a shutter control circuit controlled by saidshutter time determining circuit; and a source of supply connected tosaid two circuits, the improvement which comprises a series circuitconnected across said source and including serially connected a firstresistor and capacitor and a switching transistor with its baseelectrode connected to the juncture between said resistor and capacitor,said series circuit acting to trigger said switching transistor tooperate said shutter control circuit when said shutter time determiningcircuit fails to operate after elapse of a predetermined interval afterthe operation of said initiating means.
 2. The shutter operating circuitaccording to claim 1 wherein said photo-electromotive circuit isconnected in series with a capacitor which is normally short circuit bya switch which is opened when a shutter release is operated so that whenan object to be photographed has sufficient light, saidphoto-electromotive element charges said capacitor within apredetermined interval to trigger said high input impedance transistor.3. The shutter operating circuit according to claim 1 wherein said firstresistor is a variable resistor.
 4. The shutter operating circuitaccording to claim 1 is further provided with the second resistorconnectable across said first resistor of said series circuit and aswitch adapted to connect said second resistor in parallel with saidfirst resistor.